Methods of this general type are well known. An early disclosure is in U.S. Pat. No. 3,218,776. In that method the film is thermoformed into pouches and filled while travelling in a circular path, but flat bed systems are also known. The methods involve laying a thermoformable film on to a conveyor which has an upper surface which includes longitudinal side margins and a plurality of moulds between the side margins, holding the side edges of the film to the side margins to resist inward movement of the side edges, and thermoforming the film into the moulds to form pouches in the moulds, and then filling and sealing the pouches while on the conveyor.
Various mechanisms are known for holding the edges of the film to the side margins of the conveyor. Often the methods involve mechanical tenters or other clips by which the edges of film are mechanically secured to the upper surface of the side margins. Another method involves providing a groove along each side margin of the conveyor and pressing an elastic or other member into the groove so as to trap an edge of the film in the groove.
The known systems suffer from various problems. For instance the use of clips or tenters is liable to result in non-uniform holding forces along the edges, with consequentially increased risk of wrinkling in the moulds. Another disadvantage with mechanical clips is the difficulty of achieving rapid fastening and release, as is required for high speed manufacture of pouches. Another problem is that the clips have to be positioned for a particular film width and so any variations in film width necessitate modifying the apparatus.
All these methods also have the disadvantage that they require extra mechanical items to provide the necessary holding and this in turn necessitates careful registration and synchronisation with the high speed manufacturing process. Also they all suffer from the disadvantage that the holding system is liable to be contaminated by whatever is filled into the pouches with the result that, during long term running of a high speed process, serious contamination problems are likely to occur. This may reduce the effectiveness of the holding system or may even make it inoperable. This is a particular problem when the material which is being filled is a liquid or gel, for instance a liquid detergent.
Another method involves providing temporary adhesion between the film and the conveyor. For instance adhesion can be provided by wetting it with water when the film is water soluble. However this system has the disadvantage that if strong adhesion is provided the film may be permanently weakened, but if only weak adhesion is provided then the adhesion is insufficient to hold the edges against strong thermoforming forces.
The object behind the invention is to provide a system for holding the side edges of the film to the side margins of the conveyor to resist inward movement of the side edges during thermoforming, but which avoids some or all of the problems discussed above.